I just noticed the abnormality with Pidgin 2.5.5 (running on Debian) these few days — I couldn’t connect my Yahoo messenger account at all. According to the discussion of Bug #389278 it appears that Yahoo was trying to change its protocol.

A quick way to have Pidgin connecting to Yahoo messenger server again is to change the Pager server setting for the Yahoo account in Pidgin.

To do this, from Pidgin:

Manage Accounts (Ctrl + A)

Select the Yahoo account from the list (assuming you’ve set one up before), and click Modify

Click on Advanced tab, under the Yahoo Options, in the Pager server field replacescs.msg.yahoo.com with cn.scs.msg.yahoo.com